Cellular telephone systems provide mobility to users allowing them to maintain a presence on the cellular telephone network as they travel over a wide geographic area. More recently, data network based voice communication, for example, using Voice-over-IP (VoIP) protocols has been applied telephone communication, for example, providing telephone services to customers using VoIP based telephone devices. An approach has been proposed to coordinate cellular telephone and IP based telephony such that calls to a cellular telephone user can be directed to a IP based telephone when the user can receive calls as such a telephone, and calls can be handed off between an IP based connection and a cellular telephone based connection.
In addition, mobile telephone units are becoming available that include radio interfaces to both cellular telephone radio networks, and wireless local area networks. For example, a user can use telephone services over the cellular network while accessing data services over the wireless local area network.